1. Field of the Invention
The present invention relates to a folded back portion flattening device which flattens a folded back portion of a bundle of sheets, a sheet processor incorporating the folded back portion flattening device, and an image forming apparatus. More particularly, the present invention relates to a folded back portion flattening device capable of reducing a load that is produced when a pressing roller starts pressing a folded back portion of a bundle of sheets, a sheet processor, and an image forming apparatus.
2. Description of the Related Art
Hitherto, a predetermined number of not more than 20 sheets have been folded and formed into a booklet with a stitching/folding machine. The sheets folded with the stitching/folding machine may be, for example, sheets that are simply folded, or sheets that are folded after being saddle bound, or sheets that are folded after being bound together with an adhesive rather than thread or staples (that is, after being subjected to perfect binding).
Regardless of whether the bundle of sheets is simply folded or is folded after being bound, since the bundle of sheets is slightly resilient, portions near the folded back portion (top portion of the fold or spine) of the folded bundle of sheets bulge and become curved in a U shape. As a result, the open side of the folded sheets tends to widen. When such folded sheet bundles are stacked, they become unstable. Therefore, they tend to fall apart, as a result of which they cannot be easily conveyed or stored when stacked.
In order to overcome the aforementioned problems, a folded back portion flattening device which flattens a folded back portion of a bundle of sheets so that the sheet bundle can be placed flat on a surface is disclosed in U.S. Pat. No. 6,692,208.
A known folded back portion flattening device is shown in FIGS. 15 and 16. In a folded back portion flattening device 901, a saddle-bound sheet bundle S which is folded in two and formed into a booklet after being discharged in the direction of arrow B from a pair of folding rollers 907 with a folded back portion (spine) Sb as a leading end is temporarily received and stopped by a stopping plate 905 that can be raised and lowered (see FIG. 16A). Then, in the flattening device 901, the sheet bundle is gripped by grippers 902 and 903, and the stopping plate 905 is raised (see FIG. 16B). At this time, the folded back portion Sb protrudes from the grippers 902 and 903. The stopping plate 905 moves away from the folded back portion Sb. Thereafter, in the flattening device 901, a pressing roller 904 of a pressing unit presses the folded back portion Sb, and moves in the direction of arrow A and parallel with the folded back portion Sb. The curved folded back portion Sb is flattened by being pressed by the pressing roller 904. Lastly, in the flattening device 901, the processed sheet bundle S is loaded onto a sheet-discharge tray 908 by being discharged into the sheet-discharge tray 908 in the direction of arrow B by a pair of discharge belts 906 (see FIG. 16C).
In this way, the known folded back portion flattening device 901 flattens the folded back portion Sb of the sheet bundle S by pressing the folded back portion Sb by a predetermined amount with the pressing roller 904.
As shown in FIG. 17, the known folded back portion flattening device 901 is incorporated in a sheet processor 920 which folds a bundle of sheets. The sheet processor 920 is connected to a body 922 of an image forming apparatus 921. The image forming apparatus 921 forms images on the sheets.
In the known folded back portion flattening device 901, the folded back portion Sb of the sheet bundle is flattened by one pressing roller, that is, the pressing roller 904. Therefore, if the sheet bundle is thick or if a material that cannot be easily flattened is used when the sheet bundle is not thick, when an attempt is made to press the folded back portion Sb by a predetermined amount that is the same as that when a thin sheet bundle is pressed, a load that is produced when the pressing roller 904 starts pressing the folded back portion Sb is large. Consequently, it is difficult to flatten the folded back portion Sb.
When a sheet bundle having a folded back portion that cannot be easily flattened is flattened because the load that is produced when the pressing roller 904 starts pressing the folded back portion Sb is large, it is necessary to reduce the speed of movement of the pressing roller 904 or to repeatedly carry out the flattening with a small pressing force. Therefore, sheet processing efficiency is poor. In addition, it is necessary to convey bundles of sheets to the flattening device at an interval that is larger than necessary, thereby resulting in poor sheet processing efficiency.
Further, in the image forming apparatus having the sheet processor having a poor sheet processing efficiency, it is necessary to convey sheets to the sheet processor at an interval that is larger than necessary, thereby resulting in poor image formation efficiency.